The present invention relates to a gas turbine having a rotor with buckets cooled by a thermal medium supplied and returned in generally axial directions along and adjacent the rim of the rotor and particularly relates to seal assemblies for axially sealing tubes carrying the thermal medium at junctures between axially stacked wheels and spacers to seal between cavities insulating the tubes from the wheels and spacers and cavities surrounding the joints between the wheels and spacers.
In gas turbines, the rotor is typically formed of axially stacked wheels and spacers bolted together in alternating relationship with one another. The wheels mount circumferentially-spaced buckets about their periphery, while the spacers lie in radial opposition to nozzles, the buckets and nozzles forming part of and residing in the hot gas path in which the products of combustion flow, rotating the rotor about its axis.
In more advanced designs of gas turbines, the buckets of the first stage and preferably the first and second stages are cooled by a thermal medium which passes axially through a bore tube assembly, outwardly in radial supply tubes in the aft disk of the rotor and forwardly through a plurality of axially extending, circumferentially-spaced tubes passing through aligned openings in the stacked wheels and spacers. The spent cooling medium is returned from the first and second stages along the rim of the rotor to the aft disk by a plurality of circumferentially-spaced return tubes in communication with radial return tubes for returning the thermal medium to the bore tube assembly. It has been determined that as the rotor structure heats up during turbine start-up, the wheel and spacer rim areas move closer together due to the temperature changes. That is, the rims of the rotor heat up faster than the interior portions of the rotor where the tie bolts are located such that the rims of the wheels and spacers move toward one another. This phenomena is known as rotor clap. As the rotor structure approaches steady-state operating temperatures, this rotor clap diminishes and eventually disappears. It is essential, however, during both start-up and steady-state operations, that the seals between the cavities surrounding the tubes within the wheel and spacer openings and cavities surrounding the seal are maintained. As a consequence of these requirements, there has developed a need for a seal that is sufficiently compliant to absorb significant compressive displacement forces caused by rotor thermal growth during start-up, yet sufficiently stiff to resist substantial transverse centrifugal forces during steady-state operation. Moreover, the seal must return to its original assembled position as the compressive displacement forces are removed and the turbine approaches steady-state operation. The seal must therefore be sufficiently robust in the radial direction to support itself under high centrifugal loadings.
In accordance with a preferred embodiment of the present invention, there is provided a seal assembly for location between axially opposed faces of the wheels and spacers of the gas turbine rotor assembly and about the thermal medium carrying tubes. The seal assemblies effectively seal between the cavities in the wheels and spacers openings about the tubes and cavities external to the seals between the registering faces of the wheels and spacers while accommodating high compressive stresses during turbine start-up and high centrifugal loadings during steady-state operation. The present seal between the cavities surrounding the tubes and the cavities surrounding the seals prevents the medium in the cavities surrounding the seals, the preferred medium being bore cooling air, from entering into or exiting from the cavities surrounding the tubes. Air transfer between cavities would induce a flow through the cavities surrounding the tubes, which would then greatly increase heat transfer from the tubes to the wheels and spacers. Increased heat transfer from the tubes to the wheels and spacers contributes to rotor failure. To accomplish the foregoing, the present invention provides a seal assembly comprising a support bushing, a seal element and a seat bushing. Each support bushing is carried by a spacer adjacent an end face thereof and provides a support for the lands of the thermal medium carrying tube. The support bushing has a radially outwardly directed flange in the region of the interface between the wheels and spacers. The seat bushing is disposed in the axially registering opening through the end face of the opposed wheel and has a radial flange mounting an annular axially facing bearing surface. A frustoconical seal element extends between the support bushing and the seat bushing. Preferably, the larger diameter of the seal element bears in a recess of the seal bushing and the inner smaller diameter edge bears against the annular bearing surface of the seat bearing. A retainer flange or stop is formed along the outer margin of the flange of the support bushing to provide support for the seal element upon axial movement of the wheel and spacer toward one another whereby high compressive forces may be absorbed. As the turbine moves toward steady-state operation, the rotor clap diminishes and the seal returns to its original assembled condition. In that condition, the seal element is stiff in a radial direction to absorb the high centrifugal field caused by rotor rotation.
In a preferred embodiment according to the present invention, there is provided in a gas turbine having a rotor comprised of axially stacked rotor wheels and spacers and aligned openings spaced from an axis of rotation of the rotor, the aligned wheels and spacers receiving a tube extending in an axial direction, a seal assembly about the tube at an interface of one of the wheels and one of the spacers, comprising a generally annular seal support in part received in a first opening in one of the wheels and spacers and including a generally radially extending flange having an annular recess in axial registration with an opposed face of another of the wheel and spacers, a seat bushing in part received in a second opening of another wheel and spacer in axial opposition to the first opening and having an seat and a seal between the seal support and the bushing, the seal including an annular frustoconically-shaped element having a radially outer edge portion engaging the seal support in the recess thereof and a radially inner edge portion engaging the seat for sealing between the seal support and the seat.